1. Field of the Invention
The present invention relates to a lighting controller for a lighting device for a vehicle, and more particularly to a lighting controller for a lighting device for a vehicle constructed so as to control the lighting operation of a semiconductor light source.
2. Background Art
As a lighting device for a vehicle, lighting devices using semiconductor light emitting element such as an LED (Light Emitting Diode) as a light source have been known. On such a lighting device for a vehicle, a lighting control circuit for controlling the lighting operation of the LED is mounted.
As the lighting control circuit, for example, a lighting control circuit has been proposed in which the battery voltage of the vehicle is boosted and the boosted voltage is applied to LEDs to drive the light source having a plurality of the LEDs connected in series (see Patent Document 1).
As the LED of the lighting device for a vehicle of this type, a signal chip LED having a package in which one chip is accommodated, or a multi-chip LED having a package in which a plurality of chips are accommodated, is employed. When an abnormality due to the failure of the LEDs, for instance, a short-circuit or a disconnection of the LED is detected, a method for detecting the forward voltage Vf of the LED is employed. In this case, when the abnormality of the light source in which a plurality of single chip LEDs are connected in series is detected, an accuracy for detecting the abnormality can be increased in detecting the forward voltages Vf of the individual LEDs than in detecting the forward voltage Vf of all the LEDs (the total forward voltage Vf) connected in series.
However, when the abnormality of the muti-chip LED is detected, it is difficult to detect the forward voltages Vf of the individual chips accommodated in the package. In the multi-chip LED in which the four chips are connected in series, it is a limitation to detect the total forward voltage Vf of the four LED chips. Further, when an variation of the forward voltages Vf is considered, the abnormality is hardly accurately detected.
For instance, in the case of the multi-chip LED in which the four chips are connected in series, if the variation of the forward voltage per chip is 3 to 4 V, the variation of the forward voltage Vf of the LED package during a normal time is 12V to 16V. When one chip of the multi-chip LED having the variation of the forward voltage Vf of 16 V is failed due to a short-circuit, the total of the forward voltage Vf is 12 V. However, this value is located within a range of the variation and, thus, the multi-chip LED cannot be discriminated from a normal multi-chip LED. Accordingly, in such a case, it is impossible to detect that one chip is short-circuited.
However, in this case, when the multi-chip LEDs are previously classified and ranked in view of the forward voltage Vf because the variation of the forward voltage Vf per rank is decreased, the abnormality can be detected. As a consequence, to meet the classification or the ranking of the forward voltage Vf, the number of types of the abnormality detecting circuit is increased and the number of managing and developing processes is increased, which cause cost to be increased.
Further, when the LED fails, the forward voltage Vf of the LED chip does not necessarily become 0V, and the forward voltage Vf may be gradually lowered. For example, in a case that a supply voltage applied to a lighting control circuit is suddenly varied, a chattering phenomenon may be generated in an output path for connecting the lighting control circuit to the LED to supply a rush current to the LED and generate a current concentration on the LED; a gradual deterioration of the LED may occur due to an environmental change such as a temperature change; or a combination thereof may occur.
When the forward voltage Vf is gradually lowered to lead to the failure of the LED, the variation of the forward voltage Vf needs to be considered to accurately detect the abnormality of the LED in the direction of the short-circuit (leak). With respect to the variation of the forward voltage Vf, a “solid difference of the LED,” “temperature characteristics of the forward voltage Vf” and “V-I characteristics” are exemplified.
Thus, when the abnormality of the LED in the direction of the short-circuit (leak) direction is accurately detected, several methods may be used. For example, in one method, when a prescribed current is supplied to an electric lamp, voltage of both the ends of the electric lamp is detected and the detected voltage is compared with a previously stored voltage to detect the abnormality of the electric lamp (see Patent Document 2). In other methods, a lamp voltage when a lamp is stable or a rate of rise of the lamp voltage during an initial time to start to light is previously stored in a nonvolatile memory as an initial rate of rise of the lamp voltage, then, a lamp voltage detected during the turning on of the lamp is compared with an initial lamp voltage, or a rate of rise of the lamp voltage during the turning on of the lamp is compared with the initial rate of rise of the lamp voltage to detect the life of the lamp (see Patent Document 3).
If the forward voltage Vf is previously stored, the stored forward voltage Vf is compared with the detected forward voltage Vf so that the largest “solid difference of the LED” due to variation of the forward voltage Vf can be cancelled.
Further, when the current supplied to the LED is fixed, the variation of the forward voltage Vf due to the “V-I characteristics” can be ignored.
[Patent Document 1] JP-A-2004-51014
[Patent Document 2] JP-A-2-15597
[Patent Document 3] JP-A-10-302976